Swim Pace Calculator
Turn a 400 and 200 time trial into Critical Swim Speed, training pace zones, and clock-friendly send-offs — in yards or meters, with a printable chart for the deck. Free, instant, no signup.
CSS training zones
Sets the unit label. Times are not converted between courses — test in the course you train.
Field CSS runs ~3–6% faster than true threshold — most swimmers can’t hold it for long-rep sets. Many coaches slow it by 2–4 s/100. Applied to every pace below.
CSS = (400 − 200) ÷ (T400 − T200) = 200 ÷ 3:08 = 1:34.00 / 100 yd
| Zone | Pace / 100 | 50s @ | 100s @ | 200s @ |
|---|---|---|---|---|
| RecoveryCSS + 15s | 1:49 | 1:10 | 2:05 | 3:55 |
| AerobicEN1CSS + 10s | 1:44 | 1:05 | 1:55 | 3:40 |
| ThresholdEN2at CSS | 1:34 | 1:00 | 1:45 | 3:20 |
| VO₂ / fast aerobicEN3CSS − 5s | 1:29 | 1:05 | 1:50 | 3:20 |
| SprintSPall out — not paced from CSS | all out | — | — | — |
- Recovery.
- Easy swimming — loosen down between and after hard work.
- Aerobic (EN1).
- Steady endurance pace you can hold for long sets with controlled breathing.
- Threshold (EN2).
- “Comfortably hard” — the sustained pace of your main sets, right at CSS.
- VO₂ / fast aerobic (EN3).
- Faster than threshold for shorter reps with more rest — hard but repeatable.
- Sprint (SP).
- All-out speed with full recovery. Deliberately not paced from CSS — an aerobic test can’t prescribe sprinting.
Zone offsets and rest allowances are a common coaching convention relative to CSS — not validated physiological cutoffs. Send-offs include rest and round up to the nearest :05. Retest every 4–8 weeks.
Pace & splits
Any distance and time — pace per 100 and 50, plus an even-split table. Print it as a goal-time splits card for the deck.
| At | Split time |
|---|---|
| 50 | 39.00 |
| 100 | 1:18.00 |
| 150 | 1:57.00 |
| 200 | 2:36.00 |
| 250 | 3:15.00 |
| 300 | 3:54.00 |
| 350 | 4:33.00 |
| 400 | 5:12.00 |
| 450 | 5:51.00 |
| 500 | 6:30.00 |
Assumes perfectly even pacing — real swims usually go out faster and come home slower.
Writing the practice is the other half.
MySwimSet turns these paces into deck-ready practice cards — write a full workout in coach shorthand, print it, run it, and track your season swimmer by swimmer.
Get started freeFrequently asked
What is Critical Swim Speed (CSS)?
CSS is an estimate of the pace you could hold for a long, steady swim — roughly your threshold pace. It comes from two time trials: CSS speed = (400 − 200) ÷ (T400 − T200). The per-100 version is simply (T400 − T200) ÷ 2. It is a field estimate, and it typically runs about 3–6% faster than a lab-measured threshold, which is why this calculator includes a long-rep adjustment.
Why doesn’t the calculator convert times between yards and meters?
Course conversion factors (SCY to SCM or LCM) are per-event approximations, and strokes benefit differently from turns — a converted CSS is a guess stacked on a guess. We would rather you test in the course you actually train in and get paces you can trust. Pick SCY, SCM, or LCM so the labels match your pool.
How often should we retest CSS?
Every 4–8 weeks is the standard advice. Fitness changes over a season, and stale zones drift into junk paces. Re-run the 400 and 200 under similar conditions (same course, similar rest before each) and regenerate the chart.
What do the training zones mean?
They are a common coaching convention expressed as seconds per 100 relative to CSS — Recovery around CSS+15, Aerobic around CSS+10, Threshold at CSS, VO₂ work around CSS−5. They are useful, practical bands, not validated physiological cutoffs, and Sprint work is deliberately not paced from CSS because an aerobic test can’t prescribe all-out speed.
Is this really free?
Yes — no signup, no ads, no locked results. It’s built by MySwimSet, the swim practice planner coaches use to write deck-ready workouts and track their season. If the calculator earns your trust, the planner is the paid product behind it.